The present invention relates generally to medium access control in communication networks.
Sensor networks are used in many different applications such as, for example, habitat monitoring, location tracking and inventory management. Such networks are typically characterized by hundreds or thousands of individual sensors located at sensor nodes distributed over a desired area. The sensors in a sensor network are typically able to communicate, for example via wired or wireless communications methods, to other sensors and one or more monitoring nodes in the network. Each sensor is able to detect at least one characteristic of its surroundings by obtaining appropriate measurements, such as acoustic, temperature, seismic, or other measurements, and may be able to perform simple computations related to those measurements. Then, at a predetermined schedule or in response to an ad hoc request from, for example, a monitoring node, the sensor transmits those measurements and computations to other nodes in the network, such as to the aforementioned monitoring node.
Sensor networks face significant energy constraints because, typically, sensors are spread over a wide area and are unattended. Accordingly, replacing batteries on the sensors may be cost and time prohibitive or even impossible. While such sensors have gained significantly in processing abilities, the amount of energy stored in sensor batteries has not gained to such an extent. Therefore, many advanced techniques for routing, communications, signal processing and hardware design have been used to help reduce the required transmit power and, hence, increase the life of batteries.
One such technique uses multiple cooperating antennas to increase the throughput and/or substantially reduce the transmit power of the communication system. Such techniques may be implemented, illustratively, using well-known Multiple Input, Multiple Output (MIMO) algorithms at the physical layer (PHY) of a network to exploit phenomena such as multipath propagation to increase throughput, or reduce bit error rates, rather than attempting to eliminate effects of multipath propagation distortion. Such techniques are useful to reduce the required transmit power for a signal.
A distributed sensor network can use such multi-antenna techniques by forming cooperating sensor clusters that function as virtual antenna arrays. However, system performance is dependent not only on algorithms at the physical layer, but also is dependent on the design of higher layers, such as at the medium access control (MAC) layer, as well as the interaction between different layers. Wireless medium access control (MAC) protocols used in wireless ad hoc networks, such as IEEE 802.11, are widely deployed but typically do not effectively consider advanced physical layer (PHY) technologies such as adaptive beamforming and power control. Some prior attempts have relied on non-adaptive directional antennas or independent beamforming by a single user, which is typically unable to sufficiently account for interference when the network is densely populated. On the other hand, other prior attempts have relied on centralized control for data transmission, which is only possible with increased network overhead due to the increased data traffic between network nodes and a central control node.